Abstract
The American black bear undergoes a 3–5 month winter hibernation during which time bears do not eat, drink, defecate, or urinate. During hibernation renal function (GFR) is 16–50% of normal but urine is reabsorbed across the urinary bladder (UB) urothelium thus enabling metabolic recycling of all urinary constituents. To elucidate the mechanism(s) whereby urine is reabsorbed, we examined the UBs of five nonhibernating wild bears using light, electron (EM), and confocal immunofluorescent (IF) microscopy–concentrating on two components of the urothelial permeability barrier – the umbrella cell apical membranes and tight junctions (TJ). Bear UB has the same tissue layers (serosa, muscularis, lamina propria, urothelia) and its urothelia has the same cell layers (basal, intermediate, umbrella cells) as other mammalians. By EM, the bear apical membrane demonstrated a typical mammalian scalloped appearance with hinge and plaque regions – the latter containing an asymmetric trilaminar membrane and, on IF, uroplakins Ia, IIIa, and IIIb. The umbrella cell TJs appeared similar to those in other mammals and also contained TJ proteins occludin and claudin - 4, and not claudin –2. Thus, we were unable to demonstrate urothelial apical membrane or TJ differences between active black bears and other mammals. Expression and localization of UT-B, AQP-1 and -3, and Na+, K+-ATPase on bear urothelial membranes was similar to that of other mammals. Similar studies of urothelia of hibernating bears, including evaluation of the apical membrane lipid bilayer and GAGs layer are warranted to elucidate the mechanism(s) whereby hibernating bears reabsorb their daily urine output and thus ensure successful hibernation.
Highlights
A number of bear species, including the American black bear (Ursus americanus), undergo a winter hibernation for 4–5 months during which time bears do not eat, drink, defecate, or urinate
Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society
By light and electron microscopy, the wall of the bear urinary bladder, as in rat and other mammalian species (Hicks 1975) comprised four layers: an outermost thin serosa, a thick smooth muscle compartment, a sub-epithelial lamina propria of loose connective tissue, and an epithelial cell layer (“urothelia”) lining the bladder lumen (Fig. 1A) and separated from the subepithelial blood capillaries and lamina propria by a thin basal lamina seen on electron microscopy (Fig. 1C)
Summary
A number of bear species, including the American black bear (Ursus americanus), undergo a winter hibernation for 4–5 months during which time bears do not eat, drink, defecate, or urinate (the latter documented in: Nelson 1973 [includes earlier references], Toien et al 2011; Folk et al 1972; Nelson et al 1973; Barboza et al 1997). Despite lack of urination, hibernating bears do not become uremic and blood urea, not creatinine, concentrations fall during hibernation (Nelson et al 1973, 1984; Nelson 1978; Hellgren et al 1990; Barboza et al 1997; Tajana and Cervigni 2013). The urinary bladder catheterization in hibernating bears has yielded 24 h urine volumes and random catheterization volumes ( reflecting 24 h urine volume, see below) in the range of 70–180 mL (Brown et al 1971; Nelson et al 1973, 1975; Folk et al 1974) while standard inulin and creatinine clearance studies during hibernation revealed glomerular filtration rates (GFR) of 12–64 mL/ min – representing 16–50% of summer GFR in the same bears (Brown et al 1971).
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